1. Field of the Invention
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to an active-matrix type liquid crystal display device and a driving method of the same.
2. Description of the Related Art
Among the liquid crystal display devices, particularly the active-matrix type liquid crystal display device having TFT (Thin Film Transistor), which is an active device, provided at each pixel has been widely used for a various kinds of devices from portable devices such as portable telephones to thin-type television sets, since it is capable of providing a high image quality with a low power consumption. Comparing a television set using the liquid crystal display device with a CRT (Cathode Ray Tube) type television set, the television set using the liquid crystal display device has many advantages, e.g., capable of providing a large area with a thin model, capable of achieving high definition, and capable of being driven with a low power consumption. However, it is pointed out that contours of images become blurred when displaying moving pictures.
While there are some reasons for the blurring of the contours when displaying moving pictures, it is considered substantially because the liquid crystal display device is a hold-type display device. The hold-type is a device which employs a display method with which luminance of each pixel is held until it is rewritten to a signal of a next frame. In the meantime, CRT exhibits such a characteristic that, when an electron beam is irradiated to a phosphor surface, a phosphor in that area illuminates and the luminance decreases rapidly thereafter at a time constant. This is called an impulse type in contrast with the hold type.
In a case of the hold-type display device, signals of a previous frame are continuously displayed until signals of a next frame are written. Thus, human beings recognize the signals of the previous and following frames integrally in terms of time in a contour part of a moving image, so that the human beings perceive it as having a blurred image. There have been mainly two approaches made for overcoming the issues of the hold type. One is to increase the frame frequency, and generate and display a frame image (that is not originally present) between a previous frame and a following frame. This is called a double-speed drive, since display is conducted at a speed that is twice as fast as a normal speed. With this, change of the images between the flames is decreased, and the blur generated at the contours of images can be suppressed. The other is a method which changes a driving method so as to obtain a display characteristic close to that of the impulse type, which is a technique called a quasi-impulse drive. Comparing both methods, the double-speed drive has an issue of an increase in the cost for circuit components, since it uses a sophisticated signal processing technique such as analysis of video signals to be displayed, generation of intermediate images, etc. The other quasi-impulse drive does not require the sophisticated signal processing. However, it is necessary to have such a characteristic that is capable of writing video signals at a high speed as in that of the double-speed drive for the liquid crystal display device.
The liquid crystal display device which performs such quasi-impulse drive will be described by referring to the accompanying drawings. FIG. 27 is a block diagram and a circuit diagram showing a structural example of the liquid crystal display device which performs such quasi-impulse drive. FIG. 28 is an enlarged circuit diagram which shows a single pixel taken out from FIG. 27. Hereinafter, explanations will be provided by referring to FIG. 27 and FIG. 28. Not only the pixel which is disposed between a gate line G1 and a gate line G2 and connected to a data line D4, but also all the other pixels are referred to as pixels 910.
This liquid crystal display device is configured with a pixel matrix 901, a data driver circuit 902 for driving data lines D1-D4, and a gate driver circuit 903 for driving gate lines G1-G4. In the pixel matrix 901, the pixel 910 configured with a TFT 911 as a pixel switch, a liquid crystal capacitance Clc, and a storage capacitance Cst is arranged in matrix at each intersection point between the data lines D1-D4 and the gate lines G1-G4 which are arranged in matrix. The liquid crystal capacitance Clc is a capacitance configured with a pixel electrode 912 and a common electrode 913 disposed in each pixel 910 and a liquid crystal substance 914 disposed therebetween. The storage capacitance Cst is a capacitance that is configured with two electrodes, i.e., an electrode 915 whose one end is electrically connected to the pixel electrode 912 and an electrode 916 whose other end is connected to a wiring VCS. A voltage is applied to the wiring VCS from a constant potential power source.
Operations of the liquid crystal display device at the time of performing the quasi-impulse drive will be described by referring to a timing chart of FIG. 29. A frame period Tv corresponding to a cycle at which video signals of one screen is inputted to the liquid crystal display device from outside is divided at least into two periods Td and Tb. The period Td is a period where the video signals are written to the liquid crystal display device, and the period Tb is a period where black signals are written to the liquid crystal display device.
Next, operations in the period Td will be described. The gate driver circuit 903 performs an operation of selecting each of the gate lines G1-G4 sequentially in the period Td. For example, in a period where the gate driver circuit 903 selects the gate line G1, it is possible to write the video signals to all the pixels 910 that are connected to the gate line G1 when the data driver circuit 902 writes the signals corresponding to the video signals to each of the data lines D1-D4. Through performing this operation for all the gate lines G1-G4, the video signals for the one screen can be written to the liquid crystal display device.
The gate driver circuit 903 also performs an operation of selecting each of the gate lines G1-G4 sequentially in the period Tb. For example, in a period where the gate driver circuit 903 selects the gate line G1, it is possible to write the black signals to all the pixels 910 that are connected to the gate line G1 when the data driver circuit 902 writes the black signal to each of the data lines D1-D4. Through performing this operation for all the gate lines G1-G4, the black signals can be written to all the pixels 910 of the liquid crystal display device.
In FIG. 29, a voltage Vlc1,1 shows a voltage of the pixel 910 that is connected to the data line D1, which is disposed between the gate line G1 and the gate line G2. Similarly, a voltage Vlc1,2 shows a voltage of the pixel 910 that is connected to the data line D1, which is disposed between the gate line G2 and the gate line G3.
Through such operations, the liquid crystal display device displays the video signals in the period Td that is a first half of one-frame period, and displays black in the period Tb that is a latter half. When a response speed of the liquid crystal display device is sufficient, each of the pixels 910 of the liquid crystal display device changes to the luminance that corresponds to the written signal when the video signal is written. When the black signal is written thereafter, the luminance decreases regardless of the video signal, and black is displayed. That is, it exhibits a display characteristic that is close to the impulse type such as CRT. Therefore, it is possible to decrease blurring generated when displaying a moving picture, which is attributed to being the hold type.
However, in order to achieve the quasi-impulse drive, it is necessary to write the video signals to the liquid crystal display device at a high speed in a shorter period than the frame period and to write the black signals in a remaining period. Thus, it is necessary to operate the gate driver circuit and the data driver circuit at a high speed. Further, the video signals are written to the liquid crystal display device with a frequency that is different from the frequency of the video signals inputted to the liquid crystal display device, so that it is necessary to provide a frame memory for converting the frequency. As described, since the gate driver circuit and the data driver circuit capable of high-speed actions as well as the frame memory are required, there is such an issue that the manufacturing cost for the liquid crystal display device is increased.
An example of the liquid crystal display device which overcomes the above-described issue and achieves the quasi-impulse drive is depicted in Japanese Unexamined Patent Publication 9-127917 (pp. 3-4, FIG. 1: Patent Document 1). The liquid crystal display device depicted in Patent Document 1 is structured in such a manner that: pixels each having two TFTs are arranged in matrix at intersection points of signal lines (data lines) and scanning lines (gate lines) arranged in matrix; a black signal supplying line is disposed in parallel to each signal line (data line); a black signal supply command signal wiring is disposed in parallel to each scanning line (gate line); a gate terminal of one of the two TFTs disposed in each pixel is connected to the scanning line (gate line), and a drain terminal thereof is connected to the data line; a gate terminal of the other TFT is connected to the black signal supply command signal wiring, and a drain terminal thereof is connected to the black signal supplying wiring; and the both source terminals of the two TFTs are connected to the liquid crystal capacitance.
Next, operations will be described. Each scanning line is scanned sequentially by the gate driver in one-frame period. When the source driver supplies the video signal to each signal line by corresponding to the scanning actions, the video signal is sequentially written to the liquid crystal display device by a row unit according to the scanning. The black signal supply command signal wiring is scanned by another gate driver at a time that is shifted from the timing at which the each of the above-described scanning lines is scanned. Upon this, the potential of the black signal supplying wiring is sequentially written to the liquid crystal display device by a row unit.
As described, with this liquid crystal display device, the video signals and the black signals can be written individually at different timings by two control lines (the scanning line and the black signal supply command signal wiring) Thus, it is possible to write the video signals and the black signals with the same frequency as that of the video signals supplied to the liquid crystal display device. Therefore, the gate driver circuit and the data driver circuit simply need to operate at a normal speed, and the frame memory is not necessary. As a result, the quasi-impulse drive can be achieved at a low cost.
However, there are following issues in the liquid crystal display device of Patent Document 1. One is that the luminance of the liquid crystal display device is deteriorated, and the other is that the cost for the liquid crystal display device becomes increased for providing two gate drivers. The reasons will be described below.
The reasons for deteriorating the luminance are as follows. Typically, the liquid crystal display device provides displays through controlling a transmission light amount of light from a light source called a backlight at each pixel of the liquid crystal display device. Thus, the maximum luminance that can be displayed with the liquid crystal display device is determined according to the maximum luminance of the backlight and the maximum transmittance of the pixels of the liquid crystal display device. As one of the important factors for determining the maximum transmittance of the pixels, there is a numerical aperture. The numerical aperture herein is a ratio of an area of each pixel where the light transmits through with respect to an area that is a product of lateral and longitudinal pixel pitches which define a single pixel. Naturally, the higher the numerical aperture, the higher the maximum transmittance of the pixels becomes. As a result, the maximum luminance of the liquid crystal display device becomes increased as well.
With the liquid crystal display device of Patent Document 1, TFTs for writing black, the black signal supply command signal wiring, the black signal supplying wiring, and the like for controlling the TFTs are required in addition to the TFTs required for writing the video signal to each pixel and the wirings (scanning lines and the signal lines) for controlling the TFTs. Thus, the numerical aperture is deteriorated. Particularly, the area for the wirings cannot be reduced dramatically, unless the wirings are formed in a multilayer structure. Meanwhile, when the wirings are formed in a multilayer structure, there is generated another issue of increasing the process cost for the liquid crystal display device. Thus, it is difficult with the disclosed method to improve the luminance at a low cost.
The reasons for increasing the cost for the liquid crystal display device are as follows. Regarding the circuits which scan the gate lines and the like of the liquid crystal display device, it is typical to mount driver ICs on a substrate of the liquid crystal display device or to simultaneously fabricate the circuits on the substrate by using a same process for the pixel TFTs.
The liquid crystal display device of Patent Document 1 requires a scanning circuit for writing the black signals, in addition to the scanning circuit used for writing the normal video signals. Naturally, the cost is increased when separate driver ICs are used for the two scanning circuits. Meanwhile, even when the scanning circuits are fabricated on the substrate with TFTs, it is necessary to have an extra substrate for providing the layout for the scanning circuits. Normally, the liquid crystal display devices are manufactured by having a plurality of liquid crystal display devices arranged on a large-scale mother substrate. The process cost required for this manufacture is determined according to a unit of the mother substrate, and the cost for the individual liquid crystal display device is proportional to a value that is obtained by dividing the cost for the single mother substrate by the number of liquid crystal display devices disposed on the single mother substrate. Thus, when the area of the liquid crystal display device is increased, the number of liquid crystal display devices which can be disposed on the single mother substrate is decreased. This results in increasing the manufacturing cost. Because of the above-described reasons, the cost for the liquid crystal display device is increased with the method that requires two scanning circuits.